The invention is directed generally to the material sensing arts and more particularly to a novel and improved sensor apparatus for detecting the flow of material along a given path of travel.
While the invention may be useful in connection with monitoring the flow of various materials, the description will be facilitated by reference to the particular problem of detecting the passage of a plurality of discrete articles, such as seeds in a field seed planter, along a given path of travel. For example, such monitoring is often desired at discharge tubes of the field seed planter where the seeds are discharged to the ground to be planted. It will be understood that the invention is not so limited, but may also find utility in connection with the monitoring of flow of materials of various types through tubes, conduits or other structures which define predetermined paths of travel through which such material or seeds or other discrete articles may travel or flow and be detected.
As is well known, a farmer engaged in mechanized planting of various seeds utilizes a planting machine pulled behind a tractor. Such planting machines usually include a plurality of spaced apart individual planting units which are supplied with seeds from one or more hoppers or containers so that a plurality of rows of seeds may be planted at one time. The prior art has provided a number of different seed sensing and/or monitoring systems for maintaining a count of the seeds planted by such a planting machine and for providing useful indications or read-outs such as seed population and the like to the farmer or machine operator.
In such systems, an individual sensing apparatus or device is generally associated with each of the seed planting units to provide an output signal in response to the passage of seeds therethrough. For relatively small and densely planted seeds such as vegetable seeds or the like, individual seeds may not be detected as such, but rather it may only be desired to detect the presence or absence of a flow of seeds being delivered. In this regard, the sensing apparatus or device is usually associated with a seed planting conduit or outlet chute of the planting unit through which the seeds travel immediately prior to being released from the machine to be planted. On the other hand, where relatively larger and less densely planted seeds such as corn or soybeans are being planted, it is usually desirable to maintain a count of the seeds planted by each unit so that seed population and other useful seed count-dependent information can be determined.
The prior art has developed a number of different approaches for the design and operation of such seed sensors. The earliest of these approaches utilized a sensitive mechanical switch having an actuator member placed in the flow path of the seeds within a seed chute or conduit. Hence, each seed passing through the conduit would strike the actuator member of the mechanical switch, causing the switch to momentarily switch between open circuit and closed circuit states or vice-versa to develop a corresponding electrical signal. Such sensor devices are shown, for example in U.S. Pat. Nos. 2,907,015 to Young, 3,527,928 to Ryder et al.; and 3,632,918 to Anson et al.
Another approach to seed sensing utilizes photoelectric or photosensitive devices to detect the passage of seeds through the tubes or chutes. In such an arrangement, a light source and a photoelectric or photosensitive device are placed in optical alignment at opposite sides of the seed chute or conduit. Hence, the passage of a seed through the conduit and between the light beam and photo sensitive device produces a characteristic output signal, which output signal can then be monitored or otherwise processed to detect the flow, and in some instances to count the seeds passing through the planting tube or chute. Such devices are shown, for example in U.S. Pat. Nos. 3,537,091 to Shenkenberg; 3,723,989 to Fathauer; and 3,974,377 to Steffen. These photosensitive seed sensing devices are the type most widely used in the art today and have enjoyed widespread acceptance and commercial success.
Yet another form of sensing device has been proposed which makes use of ultrasonic sensing apparatus. This apparatus sets up an ultrasonic wave or energy rather than light energy in the tube and operates to detect disturbances in this ultrasonic energy caused by the passage of seeds through the tube. Such an ultrasonic sensing apparatus is shown in U.S. Pat. No. 3,881,353 to Fathauer.
Yet another type of seed sensor utilizes microwave energy. This type of sensor provides a waveguide intersecting a portion of the path of travel of seeds for supporting the propagation of a standing wave pattern of microwave energy. Generally speaking, this apparatus detects disturbances in this standing wave pattern due to the passage of seeds through the seed conduit, and in particular through the portion thereof in which the waveguide is located. Associated circuitry is responsive to these disturbances or changes in microwave energy in the waveguide for determining the presence or absence of seeds, as well as in some instances for counting the seeds. One such microwave seed sensor apparatus is shown for example in U.S. Pat. No. 4,246,469 to Merlo, and another such microwave seed sensing apparatus is shown in U.S. Pat. No. 4,239,010 to Amburn.
The present invention is directed to yet another form of seed sensing device which makes use of the dielectric properties of seeds and/or other material or articles flowing along a path of travel to provide for detection of such seeds, material or other articles. Generally speaking, the present invention contemplates setting up an electromagnetic field transversely of the path of travel, such as in the seed chute or conduit and detecting changes in the electromagnetic field due to the passage of such seeds or other discrete articles or the flow of material therethrough.
More particularly, in a case of seeds or articles or materials having measurable dielectric properties, we have found that a sensor having primarily capacitive or capacitance-like properties may be utilized. More particularly, a pair of relatively simple and inexpensive conductive plates may be placed to either side of the path of travel, such as a seed conduit or chute. These plates generally define plates of a capacitor with the seed conduit portions therebetween comprising the dielectric portion of the capacitor. Hence, if an object or material of a different relative permittivity or dielectric property relative to air enters this field, the electric field state will be altered. The resulting alteration can be separated into both a tansient effect and steady state effect. We have found that the transient effect is generally much less pronounced than the steady state effect and of much shorter time duration.
Additionally, a capacitor or capacitive-type of sensor arrangement, as such, is capable of essentially two basic functions. Firstly, such a device may be utilized to transmit energy; and secondly, the device may be utilized to store energy. The first or energy transmission capability is generally utilized in coupling and bypass types of uses, while the second function is useful in applications such as filters, timing, phase shifting and resonant circuit type of applications. Various prior art sensing devices utilizing capacitive type sensors are also known. Such sensors are often utilized in steady state applications to detect the amount of moisture, or some other ingredient of a material, particularly in agricultural grain moisture tester devices. Such moisture testers are shown for example in U.S. Pat. Nos. 3,794,367 to Fathauer and 4,058,766 to Vogel et al.
The prior art capacitive-type sensor and detector arrangements have utilized a number of properties of capacitors and various circuit arrangements to achieve the desired sensing or detection functions. For example, in a resonant-type of circuit, detection or sensing often utilized the phase or frequency shifts observed in the circuit. Such phase or frequency shifts occur due to the entrance of articles or materials to be detected or sensed into the electromagnetic field of the capacitive-type sensor, thus changing the properties of this field, as well as the capacitance properties of the sensor element or arrangement. Such changes of a capacitance component in a resonant-type of circuit can be detected as changes in phase or changes in the resonant frequency of the circuit.
However, we have found that the measurement of such frequency shifts or changes essentially requires narrow band FM demodulation. While this approach is workable, it requires that the detection circuits track the frequency drift of an oscillator which drives the resonant circuit, of which the capacitive sensor forms a part. We have found, however, that the oscillator drift may be expected to be as much as 10 times the detector bandwidth, which makes the problem of tracking of frequency drift relatively difficult and expensive t accomplish.
We have also found with respect to phase shift detection that a phase demodulator having a relatively high Q is needed to obtain the required sensitivity. However, a high Q circuit inherently has a very narrow bandwidth which in turn requires that suitable tuning circuits be utilized. Alternatively, various noise reduction techniques and circuits may be utilized in order to maintain an adequate signal to noise ratio to compensate for lowered sensitivity of the circuit. However, this solution is generally very costly in terms of required circuit complexity and requires relatively substantial real time for the necessary signal processing. In many detection applications, especially in the sensing of seeds which are delivered at a relatively rapid rate in a field seed planter, this amount of time for signal processing is not available.
Advantageously, we have found that amplitude detection methods may be employed, utilizing a relatively lower Q than frequency or phase detection methods. Moreover, we have found that sensitivity may be readily increased by increasing the input drive level with little increase in total noise in such an amplitude detection arrangement. More particularly, we have found that utilizing a resonant circuit such as a singly tuned, magnetically coupled circuit permits a change in amplitude to be detected as a voltage measurement directly across the capacitive sensing element.